A force sensor, also referred to as a load cell, is well known in the art and is configured for sensing an attribute of a force, e.g., magnitude or direction, applied to a physical object. A force sensor is a transducer that supplies an output signal representative of an attribute (magnitude and/or direction) of the force exerted on the physical object. A force applied to a physical object shall generally cause the physical object to deform. The deformation is a result of pressure and stress within the physical object resulting from the physical object's inertia or from a reaction force applied to the physical object by the environment. The deformation can be measured in a variety of ways. For example, one or more strain gauges are attached to a surface of the physical object to sense the deformation of the physical object at the surface. As another example, the deformation of the surface of the physical object as a result of the force applied causes a change in distance and/or orientation between a specific location on the surface of the physical object and a reference location external to the physical object. The change in distance can be measured capacitively, optically, piezoelectrically, etc.
U.S. Pat. No. 5,421,213, incorporated herein by reference, discloses a multi-dimensional force detector. The known detector comprises a disk-shaped fixed substrate and a disk-shaped flexible substrate. The fixed substrate and the flexible substrate are attached to an inner wall of a cylindrical casing along their respective peripheral portions. The fixed substrate has a first surface that accommodates a disk-shaped fixed electrode. The flexible substrate has a second surface and a third surface. The second surface accommodates four displacement electrodes. Each of the four displacement electrodes is shaped as a quadrant of a disk or of a ring. The first surface and the second surface face each other. The fixed electrode and each respective one of the displacement electrodes form a respective capacitor. The third surface faces away from the fixed substrate and accommodates a cylindrical body positioned coaxially with the disk-shaped flexible substrate. Assume that the cylindrical casing is kept stationary and that an external force is applied on the cylindrical body. The flexible substrate deforms in response to the external force. The deformation of the flexible substrate is determined by the magnitude and direction of the external force, by the reaction forces exerted by the cylindrical casing on the flexible substrate along the peripheral portion of the flexible substrate, by the elastic properties of the material of the flexible substrate, and by the shape of the flexible substrate if no external force is being applied. The deformation gives rise to a respective change in the capacitance of the respective capacitor. Measuring the respective changes in the capacitances provides information about the direction and magnitude of the external force.